Current fans in Central Air Conditioning(CAC) systems for residential homes normally operate only when the CAC system is operating in a heating mode or a cooling mode. Alternatively, the fans in the CAC systems can be left in the on mode all the time. However, such a constant running of a fan system would constitute a waste of energy and power.
In CAC systems, a central heating or air cooling unit produces heated or cooled air. Normally, the heated or cooled air is directed from the heating or cooling unit through various ducts located throughout a building in order to place the heated or cooled air at desirable locations. Blowers, fans or air-type handlers generally are used to move the heated or cooled air through the ducts. Generally, thermostats are used to actuate the heating and cooling units. For example, when the air-temperature within a structure drips below a selected level, a thermostat can be adjusted to activate a heating mode when heating is desired. Likewise, when the air-temperature within a structure rises above a selected level, the thermostat can be adjusted to activate a cooling mode when cooling is desired. The CAC system is switched off when the interior air-temperature within the structure again reaches the desired selected temperature level.
Many CAC systems for heating and cooling structures use the thermostat to simultaneously activate both the fan along with the heating or cooling unit. In these systems the thermostat is usually used to simultaneously switch off both the fan and heating/cooling unit. In some heating CAC systems, the fan may continue to run after the heating unit has been shut off usually until residual heat in the heating unit has been removed. Alternatively, in some cooling CAC systems. the fan may continue to run after the cooling unit has been shut off to remove residual cool air from the cooling unit. However, no known systems exist that control the fan itself to turn on or off based on the last time the heating or cooling or constant fan modes have been activated.
Standards enacted in 1989 by the American Society of Heating, Refrigeration and Air-conditioning Engineers(ASHRAE) such as the 62 fresh air standard now require 15 cubic feet of outside air per person which translates to approximately 0.35 air changes per hour for residential buildings. The ASHRAE further includes an air quality standard which recommends a maximum concentration of CO.sub.2 of 1000 ppm(parts per million).
The Manufactured Home Construction and Safety Standards set forth by the U.S. Department of Housing and Urban Development(HUD) has enacted new standards for manufactured homes that take effect in October of 1994. The HUD standards require fresh air ventilation systems for all manufactured housing in the United States. These ventilation systems must distribute outdoor air throughout the conditioned living space. Some ventilation systems require the installation of supply ducts separate from those of the CAC system, to distribute ventilation air. The separate ventilation supply ducts are potentially an unnecessary additional expense.